With the increase in the expenses associated with irrigation, especially in the cost of obtaining and supplying irrigation water to commercial crops, determining the moisture content of soil has become increasingly important. Various methods are known for determining the amount of moisture present in soil and such systems can include conductivity sensors and time domain transmissiometry (TDT) sensors amongst others.
Conductivity sensors suffer from a lack of precision/sensitivity in their measurements, they must be in electrical contact with the soil and they are not well suited to measuring low levels of moisture and thus they are not generally an acceptable solution.
TDT sensors are better able to accurately measure low moisture levels but, as the sensors must have relatively long sensor transmission lines/electrodes to produce accurate results, such sensors can be difficult to place in the soil.
Typically, holes must be excavated in the soil to receive TDT sensors and then the soil is backfilled around the sensor and/or sensor transmission electrodes. The need to excavate and backfill the soil increases the costs of installing TDT sensors and also results in the soil surrounding the sensor having different characteristics, i.e.—density, etc. than the bulk of the soil. Such different soil characteristics surrounding the sensors can result in less correct readings of soil characteristics.
Further, due to the need for long sensor transmission electrodes, TDT sensors provide measurements over a relatively large volume of soil provide which may be undesired in some circumstances.
Another known soil sensor is disclosed in U.S. Pat. No. 6,441,622 to Wrzesinski et al. which teaches a time domain reflectometry (TDR) soil sensor which employs a coiled sensor transmission electrode that is encircled by a cylindrical secondary electrode. The sensor is installed in the ground with the soil to be tested being received in the space between the secondary electrode and the coiled sensor electrode. As is apparent, the Wrzesinski sensor requires the sensor to be installed by excavation and backfilling of soil about and in the sensor.